Lecture 4 - Alkanes - Alkanes and Alkyl Halides Notes | EduRev

: Lecture 4 - Alkanes - Alkanes and Alkyl Halides Notes | EduRev

 Page 1


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Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 1 of 31 
Module 3  Alkanes and Alkyl Halides 
 
Lecture 4 Alkanes  
3.1 Introduction 
Alkanes are hydrocarbons i.e. compounds made of carbon and hydrogen only. There are 
two kinds of alkanes-linear and cyclic alkanes or cycloalkanes. The linear alkanes have 
the general formula C
n
H
2n+2
, while cycloalkanes have the general formula C
n
H
2n
. Besides 
the simple monocyclic cycloalkanes, there are other forms such as bicyclic and tricyclic 
cycloalkanes with bridges containing any or no carbon atom. Another special class of 
cycloalkanes is the so called spiroalkanes, which have one carbon atom common between 
two rings.  
Alkanes show both constitutional and conformational isomerism. The conformational 
isomerism manifests itself as early as the second member of the alkane family-ethane 
while constitutional isomerism first appears for butane. The issue of conformational 
isomerism is discussed separately under stereoisomerism. 
Constitutional isomerism arises due to compounds having same molecular formula but 
different bond connectivity. Butane has two constitutional isomers: n-butane and 
isobutane. Similarly, pentane has 3 constitutional isomers: n-pentane, isopentane and 
neopentane (Scheme 1).  
Me
Me
n-butane
Me Me
Me
isobutane
Me Me
Me
Me
Me
Me
Me
Me
Me
n-pentane
isopentane neopentane
Constitutional isomers of butane (C
4
H
10
)
Constitutional isomers of pentane (C
5
H
12
)
Scheme 1 
 
 
 
 
Page 2


NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 1 of 31 
Module 3  Alkanes and Alkyl Halides 
 
Lecture 4 Alkanes  
3.1 Introduction 
Alkanes are hydrocarbons i.e. compounds made of carbon and hydrogen only. There are 
two kinds of alkanes-linear and cyclic alkanes or cycloalkanes. The linear alkanes have 
the general formula C
n
H
2n+2
, while cycloalkanes have the general formula C
n
H
2n
. Besides 
the simple monocyclic cycloalkanes, there are other forms such as bicyclic and tricyclic 
cycloalkanes with bridges containing any or no carbon atom. Another special class of 
cycloalkanes is the so called spiroalkanes, which have one carbon atom common between 
two rings.  
Alkanes show both constitutional and conformational isomerism. The conformational 
isomerism manifests itself as early as the second member of the alkane family-ethane 
while constitutional isomerism first appears for butane. The issue of conformational 
isomerism is discussed separately under stereoisomerism. 
Constitutional isomerism arises due to compounds having same molecular formula but 
different bond connectivity. Butane has two constitutional isomers: n-butane and 
isobutane. Similarly, pentane has 3 constitutional isomers: n-pentane, isopentane and 
neopentane (Scheme 1).  
Me
Me
n-butane
Me Me
Me
isobutane
Me Me
Me
Me
Me
Me
Me
Me
Me
n-pentane
isopentane neopentane
Constitutional isomers of butane (C
4
H
10
)
Constitutional isomers of pentane (C
5
H
12
)
Scheme 1 
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 2 of 31 
Carbon atoms in alkanes are sometimes labeled as primary (1°), secondary (2°) and 
tertiary (3°) carbon. This classification is based on the number of carbon atoms attached 
to the carbon atom. If a carbon atom is attached to one carbon atom, then it is called a 
primary carbon. Similarly, a carbon atom which is attached to two and three other carbon 
atoms is called secondary and tertiary carbon, respectively. 
H
H
H
Me
H
Me
H H
primary
carbon
secondary 
carbon
tertiary
carbon
 
3.2 Nomenclature 
Alkanes are usually referred by their trivial names. However, the IUPAC lays down 
certain rules for the nomenclature of alkanes. These rules are: 
Rule 1.  The name of an alkane consists of two parts- a prefix and a root. The prefix will 
denote the number of carbons present in the chain. Some prefixes for common alkanes 
are given below along with the number of carbons they represent (Table 1).The root is 
always ane. The moiety generated by removing a hydrogen from an alkane is called an 
alkyl group. Its name is given based on the name of parent hydrocarbon from which it is 
derived. 
 
 
 
 
 
 
 
 
 
 
 
 
Page 3


NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 1 of 31 
Module 3  Alkanes and Alkyl Halides 
 
Lecture 4 Alkanes  
3.1 Introduction 
Alkanes are hydrocarbons i.e. compounds made of carbon and hydrogen only. There are 
two kinds of alkanes-linear and cyclic alkanes or cycloalkanes. The linear alkanes have 
the general formula C
n
H
2n+2
, while cycloalkanes have the general formula C
n
H
2n
. Besides 
the simple monocyclic cycloalkanes, there are other forms such as bicyclic and tricyclic 
cycloalkanes with bridges containing any or no carbon atom. Another special class of 
cycloalkanes is the so called spiroalkanes, which have one carbon atom common between 
two rings.  
Alkanes show both constitutional and conformational isomerism. The conformational 
isomerism manifests itself as early as the second member of the alkane family-ethane 
while constitutional isomerism first appears for butane. The issue of conformational 
isomerism is discussed separately under stereoisomerism. 
Constitutional isomerism arises due to compounds having same molecular formula but 
different bond connectivity. Butane has two constitutional isomers: n-butane and 
isobutane. Similarly, pentane has 3 constitutional isomers: n-pentane, isopentane and 
neopentane (Scheme 1).  
Me
Me
n-butane
Me Me
Me
isobutane
Me Me
Me
Me
Me
Me
Me
Me
Me
n-pentane
isopentane neopentane
Constitutional isomers of butane (C
4
H
10
)
Constitutional isomers of pentane (C
5
H
12
)
Scheme 1 
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 2 of 31 
Carbon atoms in alkanes are sometimes labeled as primary (1°), secondary (2°) and 
tertiary (3°) carbon. This classification is based on the number of carbon atoms attached 
to the carbon atom. If a carbon atom is attached to one carbon atom, then it is called a 
primary carbon. Similarly, a carbon atom which is attached to two and three other carbon 
atoms is called secondary and tertiary carbon, respectively. 
H
H
H
Me
H
Me
H H
primary
carbon
secondary 
carbon
tertiary
carbon
 
3.2 Nomenclature 
Alkanes are usually referred by their trivial names. However, the IUPAC lays down 
certain rules for the nomenclature of alkanes. These rules are: 
Rule 1.  The name of an alkane consists of two parts- a prefix and a root. The prefix will 
denote the number of carbons present in the chain. Some prefixes for common alkanes 
are given below along with the number of carbons they represent (Table 1).The root is 
always ane. The moiety generated by removing a hydrogen from an alkane is called an 
alkyl group. Its name is given based on the name of parent hydrocarbon from which it is 
derived. 
 
 
 
 
 
 
 
 
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 3 of 31 
Table 1 
Number of 
Carbons 
IUPAC Prefix Name of 
Hydrocarbon 
Name of Moiety Generated 
by Removing a  Hydrogen 
1 Meth Methane (CH
4
) Methyl(CH
3
) 
2 Eth Ethane (C
2
H
6
) Ethyl( C
2
H
5
) 
3 Prop Propane ( C
3
H
8
) Propyl(C
3
H
7
) 
4 But Butane (C
4
H
10
) Butyl(C
4
H
9
) 
5 Pent Pentane (C
5
H
12
) Pentyl( C
5
H
9
) 
  
Rule 2. In case of cycloalkanes, the word cyclo should be prefixed with the name of 
alkane. Thus, a cycloalkane containing three carbon atoms is called cyclopropane.  
Cycloheptane Cyclohexane
Cyclopropane
 
For substituted cyclohexanes, the name of the substituents is added as prefixes to the 
name of parent hydrocarbon. 
Me
Me
1-Ethyl-4-methylcyclopentane
 
 
 
 
 
Page 4


NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 1 of 31 
Module 3  Alkanes and Alkyl Halides 
 
Lecture 4 Alkanes  
3.1 Introduction 
Alkanes are hydrocarbons i.e. compounds made of carbon and hydrogen only. There are 
two kinds of alkanes-linear and cyclic alkanes or cycloalkanes. The linear alkanes have 
the general formula C
n
H
2n+2
, while cycloalkanes have the general formula C
n
H
2n
. Besides 
the simple monocyclic cycloalkanes, there are other forms such as bicyclic and tricyclic 
cycloalkanes with bridges containing any or no carbon atom. Another special class of 
cycloalkanes is the so called spiroalkanes, which have one carbon atom common between 
two rings.  
Alkanes show both constitutional and conformational isomerism. The conformational 
isomerism manifests itself as early as the second member of the alkane family-ethane 
while constitutional isomerism first appears for butane. The issue of conformational 
isomerism is discussed separately under stereoisomerism. 
Constitutional isomerism arises due to compounds having same molecular formula but 
different bond connectivity. Butane has two constitutional isomers: n-butane and 
isobutane. Similarly, pentane has 3 constitutional isomers: n-pentane, isopentane and 
neopentane (Scheme 1).  
Me
Me
n-butane
Me Me
Me
isobutane
Me Me
Me
Me
Me
Me
Me
Me
Me
n-pentane
isopentane neopentane
Constitutional isomers of butane (C
4
H
10
)
Constitutional isomers of pentane (C
5
H
12
)
Scheme 1 
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 2 of 31 
Carbon atoms in alkanes are sometimes labeled as primary (1°), secondary (2°) and 
tertiary (3°) carbon. This classification is based on the number of carbon atoms attached 
to the carbon atom. If a carbon atom is attached to one carbon atom, then it is called a 
primary carbon. Similarly, a carbon atom which is attached to two and three other carbon 
atoms is called secondary and tertiary carbon, respectively. 
H
H
H
Me
H
Me
H H
primary
carbon
secondary 
carbon
tertiary
carbon
 
3.2 Nomenclature 
Alkanes are usually referred by their trivial names. However, the IUPAC lays down 
certain rules for the nomenclature of alkanes. These rules are: 
Rule 1.  The name of an alkane consists of two parts- a prefix and a root. The prefix will 
denote the number of carbons present in the chain. Some prefixes for common alkanes 
are given below along with the number of carbons they represent (Table 1).The root is 
always ane. The moiety generated by removing a hydrogen from an alkane is called an 
alkyl group. Its name is given based on the name of parent hydrocarbon from which it is 
derived. 
 
 
 
 
 
 
 
 
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 3 of 31 
Table 1 
Number of 
Carbons 
IUPAC Prefix Name of 
Hydrocarbon 
Name of Moiety Generated 
by Removing a  Hydrogen 
1 Meth Methane (CH
4
) Methyl(CH
3
) 
2 Eth Ethane (C
2
H
6
) Ethyl( C
2
H
5
) 
3 Prop Propane ( C
3
H
8
) Propyl(C
3
H
7
) 
4 But Butane (C
4
H
10
) Butyl(C
4
H
9
) 
5 Pent Pentane (C
5
H
12
) Pentyl( C
5
H
9
) 
  
Rule 2. In case of cycloalkanes, the word cyclo should be prefixed with the name of 
alkane. Thus, a cycloalkane containing three carbon atoms is called cyclopropane.  
Cycloheptane Cyclohexane
Cyclopropane
 
For substituted cyclohexanes, the name of the substituents is added as prefixes to the 
name of parent hydrocarbon. 
Me
Me
1-Ethyl-4-methylcyclopentane
 
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 4 of 31 
Rule 3. In case of branched alkanes, the longest carbon chain gives the name of the 
parent alkane. The substituents are to be prefixed using suitable locants. If there are 
multiple same substitutents, then the prefixes di-, tri, tetra- etc are added before the 
substituent name. Thus, the name of 1 is 2,3,7,8-tetramethylnonane. 
Me
Me
Me
Me
Me Me
1
2
3
4
5
6
7
8
9
longest carbon chain
1
 
If there is more than one type of substituents, then they are usually arranged in an 
alphabetical order. Thus, in compound 2, ethyl group is mentioned before the methyl 
groups. 
5-Ethyl-2,5-dimethylheptane
Me
Me
Me
Me
Me
2
 
If more than one substituent is attached to the parent hydrocarbon, the chain is numbered 
in that direction that will result in the lowest number in the name of the compound (Rule 
of lowest locants). 
5-ethyl-3-methyloctane
but not 
4-ethyl-6 methyloctane
as 3>4
Me
Me
Me
Me
 
 
 
 
 
 
Page 5


NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 1 of 31 
Module 3  Alkanes and Alkyl Halides 
 
Lecture 4 Alkanes  
3.1 Introduction 
Alkanes are hydrocarbons i.e. compounds made of carbon and hydrogen only. There are 
two kinds of alkanes-linear and cyclic alkanes or cycloalkanes. The linear alkanes have 
the general formula C
n
H
2n+2
, while cycloalkanes have the general formula C
n
H
2n
. Besides 
the simple monocyclic cycloalkanes, there are other forms such as bicyclic and tricyclic 
cycloalkanes with bridges containing any or no carbon atom. Another special class of 
cycloalkanes is the so called spiroalkanes, which have one carbon atom common between 
two rings.  
Alkanes show both constitutional and conformational isomerism. The conformational 
isomerism manifests itself as early as the second member of the alkane family-ethane 
while constitutional isomerism first appears for butane. The issue of conformational 
isomerism is discussed separately under stereoisomerism. 
Constitutional isomerism arises due to compounds having same molecular formula but 
different bond connectivity. Butane has two constitutional isomers: n-butane and 
isobutane. Similarly, pentane has 3 constitutional isomers: n-pentane, isopentane and 
neopentane (Scheme 1).  
Me
Me
n-butane
Me Me
Me
isobutane
Me Me
Me
Me
Me
Me
Me
Me
Me
n-pentane
isopentane neopentane
Constitutional isomers of butane (C
4
H
10
)
Constitutional isomers of pentane (C
5
H
12
)
Scheme 1 
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 2 of 31 
Carbon atoms in alkanes are sometimes labeled as primary (1°), secondary (2°) and 
tertiary (3°) carbon. This classification is based on the number of carbon atoms attached 
to the carbon atom. If a carbon atom is attached to one carbon atom, then it is called a 
primary carbon. Similarly, a carbon atom which is attached to two and three other carbon 
atoms is called secondary and tertiary carbon, respectively. 
H
H
H
Me
H
Me
H H
primary
carbon
secondary 
carbon
tertiary
carbon
 
3.2 Nomenclature 
Alkanes are usually referred by their trivial names. However, the IUPAC lays down 
certain rules for the nomenclature of alkanes. These rules are: 
Rule 1.  The name of an alkane consists of two parts- a prefix and a root. The prefix will 
denote the number of carbons present in the chain. Some prefixes for common alkanes 
are given below along with the number of carbons they represent (Table 1).The root is 
always ane. The moiety generated by removing a hydrogen from an alkane is called an 
alkyl group. Its name is given based on the name of parent hydrocarbon from which it is 
derived. 
 
 
 
 
 
 
 
 
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 3 of 31 
Table 1 
Number of 
Carbons 
IUPAC Prefix Name of 
Hydrocarbon 
Name of Moiety Generated 
by Removing a  Hydrogen 
1 Meth Methane (CH
4
) Methyl(CH
3
) 
2 Eth Ethane (C
2
H
6
) Ethyl( C
2
H
5
) 
3 Prop Propane ( C
3
H
8
) Propyl(C
3
H
7
) 
4 But Butane (C
4
H
10
) Butyl(C
4
H
9
) 
5 Pent Pentane (C
5
H
12
) Pentyl( C
5
H
9
) 
  
Rule 2. In case of cycloalkanes, the word cyclo should be prefixed with the name of 
alkane. Thus, a cycloalkane containing three carbon atoms is called cyclopropane.  
Cycloheptane Cyclohexane
Cyclopropane
 
For substituted cyclohexanes, the name of the substituents is added as prefixes to the 
name of parent hydrocarbon. 
Me
Me
1-Ethyl-4-methylcyclopentane
 
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 4 of 31 
Rule 3. In case of branched alkanes, the longest carbon chain gives the name of the 
parent alkane. The substituents are to be prefixed using suitable locants. If there are 
multiple same substitutents, then the prefixes di-, tri, tetra- etc are added before the 
substituent name. Thus, the name of 1 is 2,3,7,8-tetramethylnonane. 
Me
Me
Me
Me
Me Me
1
2
3
4
5
6
7
8
9
longest carbon chain
1
 
If there is more than one type of substituents, then they are usually arranged in an 
alphabetical order. Thus, in compound 2, ethyl group is mentioned before the methyl 
groups. 
5-Ethyl-2,5-dimethylheptane
Me
Me
Me
Me
Me
2
 
If more than one substituent is attached to the parent hydrocarbon, the chain is numbered 
in that direction that will result in the lowest number in the name of the compound (Rule 
of lowest locants). 
5-ethyl-3-methyloctane
but not 
4-ethyl-6 methyloctane
as 3>4
Me
Me
Me
Me
 
 
 
 
 
 
NPTEL – Biotechnology – Cell Biology 
 
 
Joint initiative of IITs and IISc – Funded by MHRD                                                               Page 5 of 31 
Rule 4. In case of bridged cycloalkanes, the naming is to be done in the following 
manner- 
I. The number of rings is to be identified. The two rings in compounds 1 and 2 are 
depicted in different colours. The prefix bicyclo is to be added for these 
compounds. 
1
1
A
A
B
B
A
A
B
B
2
2
 
II. The number of carbon atoms in each ring is to be counted including the carbons in 
the bridge but excluding those at bridge heads. Thus, in compound 1, there are 2 
carbons in both rings A and B while only 1 carbon in the bridge. Similarly, in 
compound 2, there are 2 carbons in the ring A while 3 carbon atoms in the ring B 
but no carbons in bridge. This number of carbon atoms in each ring is written in a 
square bracket separated by a dot after the prefix bicycle in a descending order. 
1
A B
A B
2
Bic
y
clo
[
2.2.1
]
Bic
y
clo
[
3
.2.
0
]
 
III. The suffix consists of the name of alkane as described for linear alkanes. The 
number of carbons (for determining the prefix of alkane’s name) is determined by 
the formula: x + y + z + 2 where x and y are the number of carbon atoms in each 
of the rings and z is the number of carbons in the bridge. Thus, the name for 
compound 1 (x + y + z + 2 = 2+2+1+2 = 7) is bicyclo[2.2.1]heptane while that for 
compound 2 (x + y + z + 2 = 3+2+0+2 = 7)  is bicyclo[3.2.0]heptane.   
 
1
A B A B
2
Bic
y
clo
[
2.2.1
]
he
p
tane
Bic
y
clo
[
3
.2.
0
]
he
p
tane
 
 
 
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